Known carburetors of this type are so-called membrane or diaphragm carburetors and have a fuel pump driven by the pressure fluctuations in the crankcase housing of the internal combustion engine, so that the fuel is present under pressure at the inlet valve to the control chamber. The fuel present in the control chamber is drawn in through the inlet passages by the under-pressure present in the inlet pipe of the carburetor, and is supplied along with the drawn-in air as a mixture to the combustion chamber.
An opening of the inlet valve for refilling the control chamber is always brought about when an underpressure exists in the control chamber because of the withdrawn fuel, with a consequence of such underpressure being a movement of the membrane or diaphragm. This underpressure must be so great that the force of the closure spring of the inlet valve can be overcome, whereby fuel present under pressure in the fuel supply line biases the inlet valve already in an opening direction. The membrane, on that side thereof remote from the control chamber, along with a part of the carburetor housing, forms an equalizing chamber which is connected with the atmosphere.
The rate or quantity of flow through the inlet passages to the intake pipe can be regulated by adjustment or set screws, whereby the fuel supply at idling speed is undertaken for adjustment with an adjustment screw especially provided for this purpose. Since the pressure in the intake pipe drops by approximately 50% and more during acceleration of the engine, a considerable reduction of the fuel supply occurs in the acceleration phase, so that the mixture is leaner, no clean filling of the combustion chambers is assured, and the internal combustion engine does not accelerate free of disturbance. In order to try to compensate for the fuel deficiency during acceleration, the mixture is made excessively rich during idling. This, however, requires an increased fuel consumption, and can additionally lead to deposits in the combustion chamber and on the sparkplug, which can lead to operating disturbances and functional sacrifices.
Furthermore, the rate or quantity of flow through the inlet passages must be adjusted in such a way that a reliable starting of the engine is assured. A choke is provided during the starting procedure for building-up a sufficient underpressure in the intake pipe especially with very small engines; this choke covers the entire intake pipe cross section. In this connection, the choke must be fitted so closely or tightly that a sufficient underpressure is built up which, however has as a consequence that the internal combustion engine, after starting, again dies or stops due to lack of air before it has been possible to open the choke. The correct setting of the idling adjustment together with the choke is consequently a matter of experience and requires a certain manual dexterity of the operator.
During hot starting there results the problem that with an open choke, the underpressure generated in the intake pipe cannot withdraw the vapor voids forming in the fuel supply line from the carburetor system. Also, this is not always satisfactorily possible with the choke closed, so that especially during hot starting considerable starting difficulties are encountered. Additionally, the danger of excessive richness of the fuel supply is encountered with the choke closed.
With the adjustment of the idling speed, care must be taken, especially with portable internal combustion engines, that even in unfavorable conditions the filling of the combustion chamber with the prepared mixture is ensured. A satisfactory acceleration of the internal combustion engine must be assured especially even in unfavorable operating conditions. Previously, it was attempted to anticipate or prevent a possible deficient fuel flow in unfavorable operating conditions by over-rich adjustment in normal conditions. A satisfactory solution, however, cannot be attained in this way, so that an acceleration of the engine is hardly possible in unfavorable operating conditions.